Progesterone Levels Much Lower Than Last Cycle? Read This!
Progesterone is secreted in pulses and can swing 8-fold within a 90-minute window in the same healthy woman -- one study found a single day's range from 2.3 to 40.1 ng/mL. That's why a single blood draw can't be reliably compared cycle to cycle, or between different women.
I get some version of this question in my inbox a few times a month: "why is my progesterone so much lower than last cycle?" Let's talk about what's actually going on.
Progesterone doesn't hold steady -- it pulses
Progesterone is secreted in pulses, not a steady stream, which produces spikes throughout the day. That means it fluctuates a lot -- one study found levels swinging 8-fold within just a 90-minute window. And it wasn't comparing different women: highs and lows ranging from 2.3 ng/mL to 40.1 ng/mL were all recorded in a single day, in the same healthy woman.
What that means for interpreting your results
It means you could get blood drawn at 8am and get an 11 ng/mL result, then at 10am get 30 ng/mL (or the reverse) -- and both could be entirely normal for you. Ideally you'd draw blood multiple times a day and average it, but nobody wants to get stuck with a needle five times in a row. So in practice, you're almost always working with a single snapshot value, and there's no way to know whether it caught a fresh pulse or a low point in between. That's exactly why a single progesterone value isn't reliably comparable -- not between different women, and not even between your own different cycles.
References
Filicori et al. (1984). J Clin Invest. Neuroendocrine regulation of the corpus luteum in the human: evidence for pulsatile progesterone secretion. Mesen & Young (2015). Obstet Gynecol Clin North Am. Progesterone and the luteal phase: a requisite to reproduction.
Common questions
Why did my progesterone test come back so much lower than last time?
Most likely timing, not a real hormonal problem. Progesterone is released in pulses throughout the day, so a single blood draw can land near a peak or a trough purely by chance.
Should I get my progesterone tested multiple times a day?
That would give a more complete picture, but it's not practical for most people. The more useful takeaway is not to over-interpret a single number, especially compared against a different single number from another day.